1. Field of the Invention
The present invention relates to a printed wiring board mounted on an electronic device or the like.
2. Description of the Related Art
A printed wiring board mounted on an electronic device is designed with a limited area which is attributable to the size of the device. For the purpose of mounting a large number of wirings on the printed wiring board with the limited area, a multilayer printed wiring board with a multilayer structure has been generally employed. In the multilayer printed wiring board, the wirings each formed on a different layer are connected to each other through via holes (or through-holes) formed in a direction perpendicular to a surface of the printed wiring board.
In recent years, the power consumption of a semiconductor device mounted on the printed wiring board has increased, which leads to a tendency of increase in current flowing in a power supply wiring of the printed wiring board. As a result, a large current, which has been conventionally flowing only in a printed wiring board for power supply on which a power supply circuit is mounted, is also flowing in a multilayer printed wiring board that allows a high-speed signal to flow therein and is densely packed, such as a printed wiring board for a CPU or a printed wiring board for image processing.
In general, the multilayer printed wiring board high in processing speed and high in density includes a via hole having a smaller diameter and a smaller amount of current flows therein per via hole as compared with the printed wiring board for power supply on which the power supply circuit is mounted. For that reason, when a large current flows therein, the via holes are relatively easily cut. In other words, a small via hole is more easily burnt out or prevented from being conductive when a current is high.
As a countermeasure against the above-mentioned problem, “Design Wave Magazine” February 2007 discloses a connection method in which via holes are aligned in a line perpendicularly to the wiring direction on connection portions of power supply wirings on different wiring layers, as illustrated in FIG. 5A. FIG. 5A illustrates a multilayer printed wiring board 201, of which only two wiring layers are illustrated. A first power supply wiring 202 is disposed on a first wiring layer, and a second power supply wiring 203 is disposed on a second wiring layer. The first power supply wiring 202 and the second power supply wiring 203 are connected to each other through multiple via holes 210, 211, 212, and 213 that are aligned in a line. The provision of the multiple via holes enables the amount of current flowing in one via hole per unit time to be reduced, and hence the reliability of the via holes can be improved.
However, when the multiple via holes are arranged as described above, a larger overlapping area of the first power supply wiring and the second power supply wiring, which are connected to each other through the via holes, is required than that in the conventional art. Further, when the through via holes are used in the multilayer printed wiring board, all of the wiring layers are pierced, and a wiring area of wiring layers other than the first power supply wiring and the second power supply wiring is narrowed. This causes a major obstacle to higher density of the printed wiring board.
In terms of the reliability, it is desirable that the amount of current flowing in one via hole per unit time be smaller, and the number of via holes be larger. Conversely, in terms of the higher-density wiring, it is desirable that the number of via holes be smaller. Accordingly, the optimum number of via holes is the smallest natural number which is equal to or larger than a value obtained by dividing a current value required to flow in at least the power supply wiring by a current value allowed to flow in one via hole.
The via holes 210, 211, 212, and 213 are equivalent in load-bearing capability to the power supply wirings 202 and 203. However, according to the experiments carried out by the inventor of the present invention, in the case of the printed wiring board illustrated in FIG. 5A, the via hole 210 or 213 is in fact always fractured and cannot carry a current load. In the case of the real printed wiring board, unlike a notional circuit diagram, power supply wirings 202a to 202c and power supply wirings 203a to 203c are disposed between the respective adjacent via holes. Accordingly, the respective via holes are not electrically equivalent to each other with influence of the power supply wirings 202a to 202c and 203a to 203c disposed between the respective adjacent via holes, with the result that the flowing current value is varied.
A description is given of the current value of the current flowing between the respective via holes in the printed wiring board illustrated in FIG. 5A with reference to FIG. 5B being a circuit diagram equivalent to the schematic drawing of FIG. 5A. It is assumed that the current value of current flowing into the power supply wiring 202 is I, resistance values of the via holes 210, 211, 212, and 213 are rv, and resistance values of the power supply wirings 202a to 202c between the respective adjacent via holes are rx, ry, and rz, respectively Likewise, it is assumed that resistance values of the power supply wirings 203a to 203c are also rx, ry, and rz, respectively. It is assumed that the current values of current flowing in the power supply wirings 202a to 202c are ia, ib, and ic, respectively. It is assumed that the current values of current flowing in the via holes 210 to 212 are id, ie, and if, respectively. The current value of current flowing in the via hole 213 is equal to the current value ic of current flowing in the power supply wiring 202c (because these two portions of wiring, 202c and 213 are in series with no path coming into the wire or leaving it to increase or decrease the current value). The current value of current flowing in the power supply wiring 203a is equal to the current value id of current flowing in the via hole 210 for the same reason as above. It is assumed that the current values of current flowing in the power supply wirings 203b and 203c are ig and ih, respectively.
In the case of the equivalent circuit of FIG. 5B, the current value of current flowing in the via hole 210 can be obtained by the following Expression (1).
                              i          ⁢                                          ⁢          d                =                                                                                                                        4                      ⁢                                              r                        x                                            ⁢                                              r                        y                                            ⁢                                              r                        z                                                              +                                          4                      ⁢                                              r                        x                                            ⁢                                              r                        y                                            ⁢                                              r                        v                                                              +                                          2                      ⁢                                              r                        y                                            ⁢                                              r                        z                                            ⁢                                              r                        v                                                              +                                          4                      ⁢                                              r                        z                                            ⁢                                              r                        x                                            ⁢                                              r                        v                                                              +                                                                                                                                          2                      ⁢                                              r                        x                                            ⁢                                              r                        v                        2                                                              +                                          2                      ⁢                                              r                        y                                            ⁢                                              r                        v                        2                                                              +                                                                  r                        z                                            ⁢                                              r                        v                        2                                                              +                                          r                      v                      3                                                                                                                                                                                      8                      ⁢                                              r                        x                                            ⁢                                              r                        y                                            ⁢                                              r                        z                                                              +                                          8                      ⁢                                              r                        x                                            ⁢                                              r                        y                                            ⁢                                              r                        v                                                              +                                          8                      ⁢                                              r                        y                                            ⁢                                              r                        z                                            ⁢                                              r                        v                                                              +                                          8                      ⁢                                              r                        z                                            ⁢                                              r                        x                                            ⁢                                              r                        v                                                              +                                                                                                                                          6                      ⁢                                              r                        x                                            ⁢                                              r                        v                        2                                                              +                                          8                      ⁢                                              r                        y                                            ⁢                                              r                        v                        2                                                              +                                          6                      ⁢                                              r                        z                                            ⁢                                              r                        v                        2                                                              +                                          4                      ⁢                                              r                        v                        3                                                                                                                          ⁢          I                                    (                  Equation          ⁢                                          ⁢          1                )            
The current values ia, ib, and ic of current flowing in the power supply wirings 202a to 202c can be obtained by the following Expressions (2) to (4), respectively.
                              i          ⁢                                          ⁢          a                =                  I          -                      i            ⁢                                                  ⁢            d                                              (                  Equation          ⁢                                          ⁢          2                )                                          i          ⁢                                          ⁢          b                =                                                                              r                  x                                +                                  r                  v                                                            r                v                                      ⁢            I                    -                                                                      2                  ⁢                                                                          ⁢                                      r                    x                                                  +                                  2                  ⁢                                                                          ⁢                                      r                    v                                                                              r                v                                      ⁢            i            ⁢                                                  ⁢            d                                              (                  Equation          ⁢                                          ⁢          3                )                                          i          ⁢                                          ⁢          c                =                                                                              2                  ⁢                                                                          ⁢                                      r                    x                                    ⁢                                      r                    y                                                  +                                                      r                    y                                    ⁢                                      r                    v                                                  +                                  2                  ⁢                                      r                    x                                    ⁢                                      r                    v                                                  +                                  r                  v                  2                                                            r                v                2                                      ⁢            I                    -                                                                      4                  ⁢                                      r                    x                                    ⁢                                      r                    y                                                  +                                  4                  ⁢                                      r                    y                                    ⁢                                      r                    v                                                  +                                  4                  ⁢                                      r                    x                                    ⁢                                      r                    v                                                  +                                  3                  ⁢                                      r                    v                    2                                                                              r                v                2                                      ⁢            id                                              (                  Equation          ⁢                                          ⁢          4                )            
The current values ie and if of current flowing in the via holes 211 and 212 can be obtained by the following Expressions (5) and (6), respectively.
                              i          ⁢                                          ⁢          e                =                                                                              2                  ⁢                                                                          ⁢                                      r                    x                                                  +                                  r                  v                                                            r                v                                      ⁢            i            ⁢                                                  ⁢            d                    -                                                    r                x                                            r                v                                      ⁢            I                                              (                  Equation          ⁢                                          ⁢          5                )                                          i          ⁢                                          ⁢          f                ⁢                                  =                                                                              4                  ⁢                                      r                    x                                    ⁢                                      r                    y                                                  +                                  4                  ⁢                                      r                    y                                    ⁢                                      r                    v                                                  +                                  2                  ⁢                                      r                    x                                    ⁢                                      r                    v                                                  +                                  r                  v                  2                                                            r                v                2                                      ⁢            id                    -                                                                      2                  ⁢                                      r                    x                                    ⁢                                      r                    y                                                  +                                                      r                    y                                    ⁢                                      r                    v                                                  +                                                      r                    x                                    ⁢                                      r                    v                                                                              r                v                2                                      ⁢            I                                              (                  Equation          ⁢                                          ⁢          6                )            
The current values ig and ih of current flowing in the power supply wirings 203b and 203c can be obtained by the following Expressions (7) and (8), respectively.
                              i          ⁢                                          ⁢          g                =                                                                              2                  ⁢                                                                          ⁢                                      r                    x                                                  +                                  r                  v                                                            r                v                                      ⁢            i            ⁢                                                  ⁢            d                    -                                                    r                x                                            r                v                                      ⁢            I                                              (                  Equation          ⁢                                          ⁢          7                )                                          i          ⁢                                          ⁢          h                ⁢                                  =                                                                              4                  ⁢                                      r                    x                                    ⁢                                      r                    y                                                  +                                  4                  ⁢                                      r                    y                                    ⁢                                      r                    v                                                  +                                  4                  ⁢                                      r                    x                                    ⁢                                      r                    v                                                  +                                  3                  ⁢                                      r                    v                    2                                                                              r                v                2                                      ⁢            i            ⁢                                                  ⁢            d                    -                                                                      2                  ⁢                                      r                    x                                    ⁢                                      r                    y                                                  +                                                      r                    y                                    ⁢                                      r                    v                                                  +                                                      r                    x                                    ⁢                                      r                    v                                                                              r                v                2                                      ⁢            I                                              (                  Equation          ⁢                                          ⁢          8                )            
A specific example of the above is described. As illustrated in FIG. 5C, it is assumed that the current value of current flowing into the power supply wiring 202 is I=4A, inner diameters of the via holes 210, 211, 212, and 213 are Φ=0.3 mm, and resistance values of the via holes 210, 211, 212, and 213 are rv=0.36 mΩ. It is assumed that the resistance values of the power supply wirings 202a and 203a are rx=0.6 mΩ, the resistance values of the power supply wirings 202b and 203b are ry=0.6 mΩ, and the resistance values of the power supply wirings 202c and 203c are rz=0.6 mΩ. It is assumed that resistance values of the first and second power supply wirings 202 and 203 between the respective adjacent via holes are 2.0 mΩ.
When calculation is made using Expressions (1) to (8), the current values of current flowing in the via holes 210 to 213 are id=1.58 A, ie=0.42 A, if=0.42 A, and ic=1.58 A, respectively.
When it is assumed that the amount of current allowed to flow in one via hole is 1 A, a current of the amount equal to or larger than the allowed amount flows in the via holes 210 and 213 so that the via holes are disconnected (i.e. they are burnt out or fractured).